The castle was part of the monastic farm at Monkstown. The castle building formed a gate into a great yard that defended the harbour below with high walls and at least one other tower. Note a carved human head on one of the corners of the castle.
The castle is oblong with a tower at each end, and the archway under the western tower was probably used to pass from one court to another within the boundary walls of the castle. Inside the main door and inner porch is a large barrel-vaulted room originally used for the storage of grain, fish and other goods. A spiral staircase ascends to a series of rooms. On the way up a room on the left may be examined for its sturdy roof construction.